Early in the game I fired everybody who is unfit for duty till I got the best of the best for leader & governors (one famous). My mediocre admirals, generals & spies were replaced without much improvement. Starting around 2785 I no longer got fast replacements and now I miss my mediocre 4th governor.

Its now 2806 and my great leader was defeated (stupid elections ;-) ) and I fired his first replacement for poor traits. Now I am waiting the month till I can fire the next replacement. --> Should the leader really be dismissable by player without an election, a revolution or serious consequences?

Dismissing the leader gives you a new leader in a democracy after an election. Real etch-a-sketchy ;-)

Does the type of government influence leader availability? (Please direct me to any useful posts on the leading characters)

Honestly, I like the idea articulated, consequences for firing leaders. If I had an election and dismissed a leader, there should be a serious unrest penalty or possibly breakaway faction. It would be cool if I dismissed a governor, his planet or series of planets go with him. Possibly same with General or Admiral taking ships or fleets; Intelligence Agents could defect and go to another side. It need not be every time, but a small chance of one of the above happening might compel you to stick with a bad character.

I never knew you could fire characters. Silly me. Hmmm, maybe I should give it a go. I had an intelligence agent with at least -20 on everything. To be honest I wish there weren't any advisors as I feel it doesn't add much to the game, except for the ambassador and the intelligence agent. I supposed it was tacked on because a lot of people like levelling up mechanics

I agree though, it seems rather odd that you can dismiss characters, especially Leaders. I would had imagined that characters are the direct product of your empire's culture and institutions, and as such you don't have direct control over whether or not they can be dismissed.